Various devices have been proposed or put in practice for the one-side welding apparatuses of the prior art. For example, an L-shaped piece is welded onto a material to be welded, and a glassfiber tape and a backing metal of copper plate are interposed therebetween to support a base metal. In this apparatus, there will be seen disadvantages that the L-shaped piece, which is in advance welded, is removed after the welding operation and the portion from which this piece has been removed must be repaired. Another conventional embodiment is that the material to be welded is attracted by two permanent magnets holding a spring plate therebetween over a welding line, and a screw pushing up this spring plate against the material to be welded is positioned at a center of the spring plate corresponding to a welding line so that the backing metal is attached to the welding line. There being about 200 to 300 mm of space between the magnets, problems arise in the cost owing to the necessary magnets, and moreover the magnetism decreases owing to rising of the temperature of the material to be welded by the input welding heat, and the magnets at last disadvantageously drop. A further conventional embodiment is that the backing metal is fixed to the material to be welded by means of a tape in the same manner as said above. Also in this case, the attaching force of the tape decreases by the welding temperature, and therefore it is inconvenient in actual practice. Difficulties prevailing in such conventional arrangements are that attaching and detaching of said members are not easy and movement of the apparatus is difficult. To date, however, no arrangement capable of overcoming these difficulties has yet been realized.